Requesting a certificate for firebird.toot.cat from Let's Encrypt .... request was successful!

Configuring webserver to use new certificate and key .... done

Applying web server configuration .... done

Requesting a certificate for fire-bird.toot.cat from Let's Encrypt .... request was successful!

Configuring webserver to use new certificate and key .... done

Applying web server configuration .... done

...and then I did it with the proper domain, firebird-fiction.com, and this time it worked:

Requesting a certificate for firebird-fiction.com from Let's Encrypt .... request was successful!

Configuring webserver to use new certificate and key .... done

Applying web server configuration .... done

The only thing I can think is that this time I manually entered "firebird-fiction.com" for the domain instead of using the option that automatically selected both that and "www.firebird-fiction.com", and somehow that default option was breaking things.